1. Field of the Invention
The present invention relates to radiation detection equipment and a nuclear medicine diagnosis device.
2. Description of the Related Art
Recently, a nuclear medicine diagnosis device using radiation detection equipment which detects radiations such as γ rays (or gamma rays) or the like has been widely spread in medical fields. A representative nuclear medicine diagnosis device includes a gamma camera device, imaging equipment for single photon emission computed tomography (SPECT), imaging equipment for positron emission tomography (PET) or the like. Further, a demand for using radiation detection equipment is increasing in a field of homeland security, in which a dosimeter using radiation detection equipment is applied to the dirty bomb counterterrorism.
Conventionally, a radiation detector mounted on the above mentioned radiation detection equipment was produced by combining a scintillator (that is, a device of absorbing radiation energy and subsequently fluorescing) with a photomultiplier tube. However, recently much attention has been paid to a technology using a semiconductor radiation detector comprised of semiconductor crystals of, for example, cadmium telluride (CdTe), cadmium (Cd) zinc (Zn) telluride (Te), gallium arsenide (GaAs), and thallium bromide (TlBr). Such a radiation detector is used for detecting radiations such as γ rays.
A semiconductor radiation detector is a device constructed for converting a charge generated by the interaction between a radiation and a semiconductor crystal into an electrical signal. This feature allows the semiconductor radiation detector to have more efficient performance of converting electrical signals than a detector only using a scintillator. Further, this feature also facilitates the semiconductor radiation detector to be downsized.
Moreover, a semiconductor radiation detector is provided with the semiconductor crystal, a cathode formed on one surface of the semiconductor crystal, and an anode arranged on the other surface of the semiconductor crystal as opposite to the cathode. The application of a direct-current (DC) high voltage between the anode and the cathode enables a signal to be extracted from the cathode or the anode via conversion of a charge generated when a radiation such as an X ray and a γ ray enters a semiconductor crystal.
Among the above described semiconductor crystals, especially a thallium bromide crystal has a larger linear attenuation coefficient due to the photoelectric effect than other semiconductor crystals of cadmium telluride, cadmium zinc telluride, and gallium arsenide or the like. Further, the thallium bromide crystal can realize the same level of γ ray sensitivity as other semiconductor crystals, by using a thin crystal shape thereof.
Accordingly, those features of the thallium bromide crystal enable radiation detection equipment mounting a semiconductor radiation detector comprised of the thallium bromide crystal and a nuclear medicine diagnosis device using the radiation detection equipment to be more downsized than radiation detection equipment mounting other semiconductor radiation detector and a nuclear medicine diagnosis device using such a semiconductor radiation detector.
Further, a price of a thallium bromide crystal is lower than prices of other semiconductor crystals of cadmium telluride, cadmium zinc telluride, and gallium arsenide or the like. This lower price of a thallium bromide crystal allows radiation detection equipment mounting a semiconductor radiation detector comprised of the thallium bromide crystal and a nuclear medicine diagnosis device using the radiation detection equipment to be provided at lower prices than radiation detection equipment mounting other semiconductor radiation detector and a nuclear medicine diagnosis device using such a semiconductor radiation detector.
Conventionally, gold, platinum and palladium or the like have been used for the materials of an anode and a cathode in a thallium bromide based radiation detector. When a bias voltage is applied to a thallium bromide based detector of which anode and cathode are made of gold, platinum and palladium or the like thereby to operate the detector over a long time, positive ions such as Tl+ (or thallium ions) tend to be accumulated near the cathode, and negative ions such as Br− (or bromide ions) tend to be accumulated near the anode.
Hereby, if such ions are accumulated near the anode and the cathode, that is, if charges are accumulated near the anode and the cathode (namely, if a phenomenon of polarization is caused), a reversed voltage opposite to the applied bias voltage is to be generated, leading to the deterioration of the energy resolution.
However, a recent technology has demonstrated that the usage of thallium layers for causing the formation reactions of a thallium metal and thallium bromide prevents the polarization near a cathode and an anode from occurring, so as to realize the operation stability of the detector. Herein, the thallium layers are respectively inserted between a general cathode and a thallium bromide crystal, and between a general anode and a thallium bromide crystal, in a semiconductor radiation detector comprised of a thallium bromide crystal (for example, see Patent Document 1 and Non-patent Document 1).
That is, an electrochemical reaction of “Tl++e−→Tl” occurs near a cathode, and the other electrochemical reaction of “Br−+Tl→TlBr+e−” occurs near an anode. Those reactions may cancel the accumulation of the ions near the cathode and the anode.
Note a term of polarization means a bias phenomenon occurring about a crystal structure, a charge or characteristics, and will be explained more specifically hereinafter.
Further, in addition to the technology of inserting the thallium layers between a cathode and a thallium bromide crystal, and between an anode and a thallium bromide crystal, in a thallium bromide based radiation detector, another technology is also developed.
Such a technology has demonstrated that periodically reversed polarities of the bias voltage applied to a detector for collecting charges enable the detector to be used over a long time. Herein, the periodically reversed polarities are generated per predetermined time in the range from 24 or less hr to 2 or more hr.
The above mentioned technology utilizes a phenomenon that the formation reactions of a thallium metal and thallium bromide are reversible reactions.